If the valve is installed correctly and you're narrowing the diameter of the return pipe (with valve), the overflow should be supplying more than you can return. Also check to see if you have enough water in the tank. Without enough water the pump still needs to supply the tank via sump water, till it reaches the overflow openings.
Easy way to gauge this is to turn off the pump. Make sure you have maybe 3-4" of water level above pump. And top off the tank water till it's just at the openings of the overflow. So now when you turn on the pump there's a lot less water displacement from the sump to the tank to get into the overflow.